The performance and emission characteristics of an internal combustion engine largely depend on the timing of the valve events, i.e., the effective opening area for fluid flow and duration. The variation of the timing by even several engine crank angle degrees significantly affects the engine performance and emission characteristics. In production engines the valve operating pattern is set at the factory and cannot be varied by the engine operator. The determination of the valve opening distance (lift) and its timing is made by taking into account various factors, e.g., performance, engine operation speed, emission and design limits. Due to the recent demand for better engine operation, advanced control technology is being applied to many of the engine systems, including the fuel/air preparation system as a function of exhaust oxygen content, engine speed, intake manifold vacuum, etc. This control is presently being performed by employing microprocessors. Control of engine systems may be greatly enhanced by the additional control of the valve events.
Even without considering the wide use of variable valve timing (VVT) in production engines, it is highly desirable to make VVT available during the engine development stage so that the search for an optimum valve train can be facilitated. Engineers can use the VVT to find the camshaft pattern that meets the design criteria without resorting to the trial and error approach to reach the goal.
Among the other areas of VVT application is automobile racing where wide ranges of engine speeds are encountered. Since for each engine speed there is an optimum set of valve operating conditions, an engine equipped with a VVT device could be controlled to run at its peak efficiency throughout its entire operating range. This may enable the engine operator to reduce the number of gear changes presently required.
The area in which the VVT would have a great impact is in the enhancement of passenger car performance and emissions. A car equipped with a VVT engine can be operated at high efficiency in a wide range of engine speeds and loads. The VVT is expected to enable the engine operator to cause the same automobile to be operated with great economy and to achieve high performance by a simple shift of a VVT control unit. It has been found that such a design could lower specific fuel consumption, lower cylinder gas temperatures, increase the turbulence intensity and burning speed of the combustion gases, and reduce the NOx production of the engine.
The control of the effective valve opening pattern may be achieved by varying the closing position and the net lift of the engine's intake valve. This is the prime objective of the VVT device. In today's conventional engines, the intake valve generally opens at about 20 to 30 degrees before top dead center and closes at about 75 degrees after bottom dead center, and opens to a maximum lift of about 0.375 to 0.425 inches. As previously noted, the operating characteristics of the camshaft remain constant over the entire driving condition. Because of the fixed timing of valve events with respect to engine crank angle, conventional engines must employ a throttled carburetor to attain variable power output. Inherent in carburetor throttling is some degree of pumping loss. Pumping loss is the combination of work necessary to overcome both the frictional losses due to air flowing around the throttle plates, and the P-V work encountered when the cylinder volume increases at subatmospheric pressures.
The VVT engine eliminates the use of the throttle plate in its carburetor. Instead, the engine load control is accomplished by varying the closing position of the intake valve. This may be called intake valve throttling (IVT) since it is achieved by early intake valve closing. With intake valve throttling the fresh charge is inducted through an unrestricted carburetor at near atmospheric pressure by the downward moving piston, and when the correct amount of fuel and air has been introduced into the cylinder, the intake valve closes. Intake valve throttling does not completely eliminate pumping loss but it does cut it down considerably. It has been reported that at intermediate speeds, the pumping loss at full load consumes about 5% of the indicated power, whereas the pumping loss at light load consumes about 50% of the indicated power. FIGS. 1A and 1B are graphs of pressure VS volume for conventional and VVT engines respectively. It may be seen that there is considerably reduced pumping loss (the crosshatched area) in the VVT engine.
FIGS. 2, 3 and 4 are graphs illustrating three different VVT schemes, which plot intake valve lift VS crankshaft position, for intake valve throttling. The valve performance shown in FIG. 3 corresponds to the P-V diagram in FIG. 1B. This method is believed to result in the greatest reduction in pumping loss. Valve performance in FIG. 2 also reduces pumping loss but not quite as much due to the fact that at light loads the engine will incur pumping loss at the beginning of the cycle as well as at the end of the cycle. The valve performance in FIG. 4 gives little reduction in pumping loss since the partially opened valve acts to constrict the flow of fuel and air into the cylinder and thus drops the cylinder pressure below atmospheric throughout the entire intake stroke. It has been shown that at reduced intake valve lift, turbulence intensity is increased, leading to a more complete burning of the fuel-air mixture and thus allowing the idle fuel mixture to be leaned out with minimum misfire and cyclic variations. It has been noted that significant gains in B.S.F.C. (brake specific fuel consumption) are possible near idle engine operation for intake valve throttling when sufficient dilution (leaning) of the mixture is employed to decrease the burnrate to be equivalent to conventional engine burnrate.
In terms of power output and unit cycle, the optimum intake valve closing position is a direct function of engine speed, the faster the engine is running and the later the intake valve should close. This utilizes the inertia of the mixture column in the induction duct, thus offsetting the cylinder pressure. As shown in FIG. 5, which graphs unit air charge VS average piston speed and corresponding camshaft angle VS valve condition, camshaft A is the most efficient in mixture induction at very low speed operation, but it falls off quite rapidly at even moderate speeds. Camshaft C is optimum for midrange speeds and likewise, camshaft F is appropriate for high speed engine conditions, but none of them are efficient throughout the entire operating range of engine speed. In a successfully controlled VVT engine, the control system, e.g., a microprocessor, should continuously vary the intake valve closing position to achieve maximum cylinder filling at all engine speeds. An engine that is mostly operated in a low speed economy mode can thereby produce more torque for uphill driving. This would allow the car manufacturers to produce their economy cars with even smaller engines or produce an economical car with more top end power and a greater top speed.
In addition to performance improvements, a decrease in the formation of NOx pollutants has been shown in intake valve throttled engines. It has been found that a VVT engine can produce roughly 24% less NOx pollutants at half load operation than a conventionally throttled engine due to the lower cylinder gas temperatures found with early intake valve closing.
For a VVT design to be successful, it most of all must be sturdy and reliable. It must be able to survive a wide range of operating temperatures and it must be able to handle the grease, oil, and fuel found in an engine compartment. It must also be able to withstand prolonged engine vibrations. In addition to these mechanical attributes, it must be able to perform its primary function of varying both valve lift and duration consistently, and it should be capable of microprocessor control.